Palette & gradient style controls + new docs site

[texodus]

This PR adds two new declarative control types, Palette and GradientStops, replacing single-color and two-stop gradient pickers where plugins declare them.

A multi-stop gradient selector with draggable stops, double-click to insert a stop sampled from the gradient, and a discrete mode that pins exactly two stops (end colors locked, shown with a lock affordance). Loading a longer named gradient into a discrete control fits it to a two-stop pair.

A palette selector with swatch editing, one-click add (opens the native color picker on the new swatch), and pointer-drag reordering with a live preview of the resulting order.

User-selected colors, palettes, and gradients are persisted through the existing per-column config maps — no new top-level config fields per control:

• Each value lives in columns_config under the plugin-declared key for that control (e.g. color, palette, gradient), keyed by column name — in the ViewerConfig emitted by a panel's save(), and per-panel inside WorkspaceConfig.panels from saveWorkspace(). These values' shape is defined by the active plugin's column_config_schema().
• The wire format is a canonical CSS string: colors as #rrggbb, palettes as linear-gradient(to right, #rrggbb, …) (positions forbidden), and gradients as linear-gradient(to right, #rrggbb 0%, …).
• Instead of a literal, a value may be a named-scale reference - var(--var-name) - pointing at a scale defined by the active theme's CSS, so a config can follow the theme rather than freeze literal colors.
• Values that match the plugin's declared default (compared after canonicalization) are stripped on save, so configs carry only user deviations and un-styled columns keep tracking theme defaults across theme switches.
• WorkspaceConfig additionally gains a top-level palette map - named color-scale definitions (--psp-user--<kind>-<name> -> canonical CSS value) shared by every panel.

New docs site. docs/ is rebuilt as a single-page demo application, and examples/blocks is deleted with its examples ported into the site's project corpus (~127 projects):

• Multi-panel <perspective-viewer> stage with a sidebar, project gallery (modal + deep links via ?project=<id>), and a theme toggle.
• A "Create Data Source" modal crossing source (Fetch / File / S3) with engine (perspective-server WASM / DuckDB-WASM), including an in-browser AWS SigV4 implementation so any S3-compatible store (AWS, R2, MinIO, …) works as a bucket browser with presigned-URL reads.
• A DuckDB SQL drawer/REPL whose results open as viewer panels, and an agent configuration dialog over the viewer's agent API.
• Streaming demos (market simulation on a fixed-size keyed ring, webcam) and a screenshot/collage build pipeline that generates gallery thumbnails and the README collage per theme.
• New viewer events perspective-layout-update and perspective-active-panel-update, emitted once per settled batch of workspace mutations.

Fixes

• Engine (C++)
  • sum aggregate no longer accumulates incorrectly when updates flip values to or from null (Fixes Handling null updates in sum aggregate #1256).
  • Expression validation no longer crashes on malformed ExprTk input (infinite loops and a buffer overflow in computed_expression.cpp).
  • column_paths / viewport off-by-one in view.cpp that corrupted region copy and windowed reads (Fixes region copy not working #3216).
• Virtual servers: window/column handling fixes in the generic SQL model (table_make_view), applied across the JS and Python DuckDB and ClickHouse adapters.
• Viewer
  • Partial-restore race between settings toggling and render tasks; restore({settings}) no longer poisons the settings flag.
  • Initial/default theme selection, settings-via-restore behavior, and missing stack frames in viewer error reports.
  • Column pin regression in panel presizing/snapshots.
• Datagrid: bar and label-bar render glitch; profile-driven scrolling optimizations across the style-handler pipeline.
• Charts: type-coercion bugs in candlestick, heatmap, series, and tick layout; rendering now composites on a worker thread via an extended renderer transport protocol.